A so-called electrophotographic optical printer is known as a printing device. The optical printer is a device for developing an electrostatic latent image formed on a photoreceptor into a visible image with toner which has been charged beforehand by a developing device and for transferring the visible image to a transfer member. The electrostatic latent image is formed on the photoreceptor by exposing or scanning the photoreceptor using a laser beam modulated according to information input from a computer.
The developing methods used for a laser printer like the optical printer are roughly classified into two types, namely a two-component developing method and a one-component developing method. The two-component developing method uses a two-component developer containing carrier and non-magnetic toner or magnetic toner. The one-component developing method employs a one-component developer consisting of non-magnetic toner or magnetic toner.
In general, the two-component developing method requires a toner-concentration detecting sensor for controlling the mixing ratio of toner and carrier, and an agitator for mixing the toner and carrier. Consequently, the two-component developing method suffers from such disadvantages that the number of parts increases and the developing device becomes larger in size. Taking a reduction in the size of the developing device into consideration, a developing device employing the one-component developing method is very advantageous, and some one-component developing methods have been put to practical use in recent years.
In developing devices adopting a one-component developing method which have been recently put to practical use, a toner layer thickness regulating member is usually provided. The toner layer thickness regulating member regulates the thickness of a layer of toner on the developer carrier when charging the toner and applying the charged toner to the developer carrier. The regulating systems for regulating the thickness of the toner layer are classified into an elastic-body-using system and a rigid-body-using system according to the material of the toner layer thickness regulating member. The elastic-body-using system has a structure in which the toner layer thickness regulating member is formed by a blade having steel-like elasticity (energy elasticity) or rubber-like elasticity (entropy elasticity). On the other hand, the rigid-body-using system has a structure in which the toner layer thickness regulating member is formed by a rod or roller made of a resin with rigidity or a metal.
For example, Japanese Publications of Examined Patent Application No. 15580/1988 (Tokukosho 63-15580) and No. 73152/1992 (Tokukosho 4-73152) disclose an elastic trailing contact method that is one of the elastic-body-using systems. In this contact method, as shown in FIG. 13, an elastic member 52 is disposed so that its body part comes into contact with a development roller 51 as the developer carrier at a contact point Q' and that two wedge gaps are formed between the development roller 51 and the elastic member 52, one gap formed on an upstream side of the contact point Q' with respect to the rotating direction of the development roller 51 (the direction of the arrow D in FIG. 13) being larger than the other gap formed on a downstream side.
An end of the elastic member 52 located on the upstream side with respect to the rotating direction of the development roller 51 is fixed, while the other end located on the downstream side is a free end. The elastic member 52 includes a flat-plate-shaped flat spring with steel-like elasticity made of a metal, etc., and a soft elastic body (elastic body with rubber-like elasticity) made of rubber, plastics, etc.
In this structure, since a relatively thick toner layer is formed on the development roller 51, an image developed by the development roller 51 can have good image density. Moreover, since the elasticity of the flat-plate-like flat spring and of the soft elastic body of the elastic member 52 can be relatively easily selected, the degree of freedom in designing the elastic member 52 is high. It is thus possible to provide an optimum elastic member 52 for each developing device.
Additionally, an elastic counter contact method that is one of the elastic-body-using systems is disclosed in, for example, U.S. Pat. No. 4,458,627 (corresponding to Japanese Publication of Examined Patent Application No. 16736/1988). In this method, as shown in FIG. 14, an elastic regulating plate 53 is disposed so that its body comes into contact with the development roller 51 at the contact point Q' and that two wedge gaps are formed between the development roller 51 and the elastic regulating plate 53, one gap located on a downstream side of the contact point Q' with respect to the rotating direction of the development roller 51 (the direction of the arrow D in FIG. 14) being larger than the other gap formed on an upstream side.
An end of the elastic regulating plate 53 located on the upstream side with respect to the rotating direction of the development roller 51 is a free end, while an end thereof located on the downstream side is fixed. The elastic regulating plate 53 is made of a metallic plate with high elasticity, rubber plate, or the like.
In this structure, since a relatively large pressure is exerted at the contact point Q', it is possible to form a relatively thin toner layer on the development roller 51 as compared to the above-mentioned elastic trailing contact method, thereby providing a fine toner image.
On the other hand, for example, U.S. Pat. No. 3,731,146 (corresponding to Japanese Publication of Examined Patent Application No. 36070/1976) discloses a rigid bar contact method as one of the rigid-body-using system. In this contact method, as shown in FIG. 15, the toner layer thickness regulating member is formed by at least one rod-like rigid bar 54. In this method, residual toner or excessive toner remaining on the development roller 51 can be removed by pressing the rigid bar 54 against the development roller 51 using a spring (not shown).
Moreover, for example, Japanese Publication of Examined Patent Application No. 22352/1985 (Tokukosho 60-22352) discloses a rigid roller contact method that is also the rigid-body-using system. In this method, as shown in FIG. 16, a friction-type charging roller 55 made of a resin having rigidity and high frictional resistance is pressed against the development roller 51. In this method, the toner can be uniformly charged without lowering the charging efficiency. Additionally, since the toner is charged by charges accumulated on the friction-type charging roller 55 to have the same polarity as the charges, good development is performed without causing cohesion of toner particles.
In a developing device using one-component developer, particularly non-magnetic one-component developer, the toner cannot be transported and supplied to the developer carrier by magnetic force. Therefore, various methods have been proposed to supply and apply the toner to the developer carrier by pressing a toner supply member with elasticity against the developer carrier and rotating the toner supply member.
For example, in the method disclosed in Japanese Publication of Examined Patent Application No. 16025/1991 (Tokukohei 3-16025), an equalizing member for supplying toner to the developer carrier and collecting residual toner from the developer carrier is pressed against the developer carrier with a contact depth ranging from 0.3 to 2.0 mm, and the equalizing member and the developer carrier are rotated in the opposite directions so that they move in the same direction as the contact section thereof.
In this structure, since the equalizing member is pressed against the developer carrier with a contact depth in the above-mentioned range, the equalizing member and the developer carrier are rubbed uniformly. As a result, the toner is uniformly supplied to the developer carrier with stability.
Moreover, for example, Japanese Publication of Examined Patent Application No. 16210/1994 (Tokukohei 6-16210) discloses a structure in which an elastic foam roller with a porous surface which is rotated in the same direction as the developer carrier is provided as the toner supply member in contact with the developer carrier.
In this structure, since the developer carrier and the elastic foam roller are rotated in the same direction, the surface of the developer carrier and the surface of the elastic foam roller move in the opposite directions at the contact section thereof. Consequently, the toner is transported to the developer carrier from the elastic foam roller in such a manner that the toner is rubbed against the developer carrier, the rubbing force on the toner particle surface increases, and the ability of applying charges produced by friction to the toner is enhanced. Namely, the amount of charges on the toner is increased by the rotations and frictions of the developer carrier and the elastic foam roller. In this structure, therefore, even when the contact depth is small, the toner can be supplied and applied to the developer carrier because of the increased amount of charges and the electrical adhesion force of the toner.
Among the above-mentioned methods, the elastic trailing contact method shown in FIG. 13 applies the lowest pressure to the toner. However, in this method, since the amount of the toner flowing to the contact point Q' is increased, excessive toner passes through between the development roller 51 and the elastic member 52.
On the other hand, in the elastic counter contact method shown in FIG. 14, since the elastic regulating plate 53 for preventing the toner from flowing to the contact point Q' is provided, the amount of toner supplied to the development roller 51 tends to be shorter than a desired amount. Therefore, in the elastic-body-using system, although less pressure is applied to the toner compared to the rigid-body-using system, the stress produced in the peripheral members tends to increase. As a result, such a problem arises that it is difficult to form a toner layer with a uniform thickness.
Furthermore, the uniformity of the thickness of the toner layer in the axis direction of the development roller 51 is higher in the elastic-body-using system than in the rigid-body-using system. However, when the elastic member 52 and the elastic regulating plate 53 are not positioned with high precision, the length (protrusion) from the contact point Q' of the elastic member 52 or the elastic regulating plate 53 to the free end, and the amount of displacement set for the elastic member 52 or the elastic regulating plate 53 vary. Thus, the structure of the elastic-body-using system suffers from a problem that the absolute value of the charge of the toner or the thickness of the toner layer easily changes.
FIG. 17 shows the ratio of the surface potential of the toner layer in the axis direction of the development roller 51 (see FIGS. 15 and 16) to the maximum electric potential (hereinafter just referred to as the "potential ratio) when the thickness of the toner layer is regulated using the elastic bar 54 (see FIG. 15) and the friction-type charging roller 55 (see FIG. 16). In FIG. 17, the curve b1 corresponds to the elastic bar 54, and the curve b2 corresponds to the friction-type charging roller 55.
When the rigid bar 54 is used, the force for regulating the toner layer decreases at the center of the development roller 51 in the axis direction (hereinafter referred to as the roller center). This would be caused by a flexure of the development roller 51 which is produced when the rigid bar 54 is pressed against the development roller 51. As a result, the thickness of the toner layer increases at the roller center, and the potential ratio becomes higher at the roller center as shown by the curve b1 of FIG. 17.
On the other hand, when the friction-type charging roller 55 is used, the force for regulating the toner layer becomes weaker at the ends of the surface of the development roller 51 in the axis direction (hereinafter referred to as the roller ends).
This would be caused by poor setting precision of the development roller 51 and the friction-type charging roller 55, i.e., the axis of the development roller 51 and the axis of the friction-type charging roller 55 are not level with each other. Consequently, the thickness of the toner layer increases at the roller ends, and the potential ratio becomes higher at th e roller ends as shown by the curve b2 of FIG. 17.
Thus, in the rigid-body-using system, if flexure of the members is produced or the setting precision of the respective members is low, the thickness of the toner layer formed on the development roller 51 varies. Such a problem is solved by forming the rigid bar 54 and the friction-type charging roller 55 by highly rigid members with sufficient flatness or cylindricity, and positioning these members with high precision. However, in actuality, it is difficult to position these members with high precision. Consequently, there is a problem that it is difficult to form the toner layer with a uniform thickness along the axis direction of the development roller 51.
Since the above-mentioned rigid-body-using system uses a rigid body as the toner layer thickness regulating member, it has a longer life compared to the elastic-body-using system. However, since the rigid-body-using system applies high pressure to the toner, it causes toner filming. It is thus difficult to certainly retain the life of the toner.
In order to prevent toner filming, various attempts have been made. In these attempts, toner is formed by a resin which has an excellent crushable property and is hard to set so as to, for example, increase the glass transition temperature Tg to a sufficiently high temperature, increase the particle diameter of the toner particles, or decrease the ratio of fine powder. However, toner with a high glass transition temperature Tg does not have good adhesiveness, and toner with a large particle diameter tends to degrade the image quality. Hence, the above-mentioned rigid-body-using system suffers from such problems that it is impossible to surely prevent the filming of the toner and certainly retain the life of the toner.
Additionally, in the structure disclosed in Japanese Publication of Examined Patent Application No. 16025/1991 (Tokukohei 3-16025), since the equalizing member comes into contact with the developer carrier while rotating in a direction opposite to the developer carrier, it is necessary to keep a sufficient contact depth and increase the toner application pressure in order to feed a sufficient amount of toner to the developer carrier. As a result, a compressive force is always exerted on the toner at the contact section, and filming of the toner occurs when the toner is used over a long period of time.
On the other hand, in the structure disclosed in Japanese Publication of Examined Patent Application No. 16210/1994 (Tokukohei 6-16210), since the elastic foam roller and the developer carrier are rotated in the same direction, the searing force acting on the toner becomes higher as the rubbing force exerted on the toner increases. Consequently, when the toner is used over a long period of time, a charge control agent and a superplasticizer which are added to the toner are separated.
Therefore, in a developing device using a non-magnetic one-component toner, in order to prevent the above-mentioned problems, it is preferred to dispose the developer carrier out of contact with the toner supply member like the equalizing member or elastic foam roller. However, in this case, when performing a low-speed development in which the developer carrier moves at a low speed, the supply of toner is carried out to follow the movement of the developer carrier. On the other hand, in a high-speed development, the supply of toner is not performed to follow a high speed movement of the developer carrier. As a result, a shortage of toner supply occurs, and therefore good development cannot be carried out.